The present invention relates to downhole separation, compression, and reinjection of a portion of the gas from a production stream produced from a subterranean zone and in one aspect relates to a method and subsurface system (SPARC) for separating gas from a production stream wherein the separated gas is compressed and reinjected by a downhole turbine-compressor unit of a SPARC which includes controls which, in turn, allow the entire production stream to initially bypass the turbine-compressor unit of the SPARC during start-up of production.
It is well known that many hydrocarbon reservoirs produce extremely large volumes of gas along with crude oil and other formation fluids, e.g. water. In such production, it is not unusual to experience gas-to-oil ratios (GOR) as high as 25,000 standard cubic feet per barrel (scf/bbl.) or greater. As a result, large volumes of gas must be separated from the liquids before the liquids are moved on to market or storage. Where the production sites are convenient to end users, this gas is a valuable asset when demands for the gas are high. However, when demands are low or when a producing reservoir is located in a remote area, large volumes of produced gas can present major problems if the produced gas can not be timely and properly disposed of
Where there is no demand for the produced gas, it is common to xe2x80x9creinjectxe2x80x9d the gas into a suitable, subterranean formation. For example, the gas may be injected back into the xe2x80x9cgas capxe2x80x9d of a production zone to maintain pressure within the reservoir and thereby increase the ultimate liquid recovery therefrom. In other applications, the gas may be injected into a producing formation through an injection well to drive the hydrocarbons towards a production well. Further, the produced gas may be injected and xe2x80x9cstoredxe2x80x9d in an appropriate formation from which it can be recovered later when the situation changes.
To separate and re-inject the gas, large surface facilities are normally required at or near the production site. These facilities are expensive due, in part, to the high-horsepower, gas compressor train(s) needed to handle, compress and inject the large volumes of gas. It follows that significant cost savings can be realized if these compressor-horsepower requirements can be reduced.
Recently, techniques have been proposed for significantly reducing the amounts of gas that need to be handled at the surface. Several of these techniques involve the use of a subsurface processing and reinjection compressor unit (SPARC) which is positioned downhole in the wellbore to separate at least a portion of the gas before the production stream is produced to the surface. A typical SPARC is comprised of an auger separator and a turbine-driven compressor unit. Gas is separated from the production stream as the stream passes through the auger and is fed into the compressor which, in turn, is driven by a turbine; the turbine being driven by the production stream, itself.
The compressed gas can then either be injected directly into a designated formation (e.g. gas cap) adjacent the wellbore or be brought to the surface through a separate flowpath for further handling. For examples of such SPARCs and how each operates, see U.S. Pat. No. 5,794,697, 6,026,901, 6,035,934, and 6,189,614.
Unfortunately, the turbine-compressor unit of a typical SPARC is subject to xe2x80x9csurgingxe2x80x9d during the start-up period of a production well. That is, a typical production stream almost always contains slugs of liquid when the well is first brought on stream, either initially or after a well has been shut-in for some period. These liquid slugs will cause the turbine/compressor to fluctuate and operate at critical shaft speeds for extended periods which, in turn, can cause severe damage to the turbine-compressor and significantly shorten the operational life of the SPARC. Accordingly, it is desirable to bypass the turbine/compressor during the start-up period of a well until the surging in the production stream has subsided and the composition of the production stream has steadied out.
The present invention provides a subsurface system for producing a mixed gas-oil stream to the surface from a subterranean zone through a wellbore wherein at least a portion of the contained gas is separated from said mixed gas-oil stream downhole and is compressed to produce a compressed gas which is re-injected into a formation adjacent the wellbore. As will be understood in the art, the production stream will likely also include some water and some solids (e.g. sand, debris, etc.) which will be produced with the oil and gas so, as used herein, xe2x80x9cmixed gas-oil stream(s)xe2x80x9d is intended to include such production streams.
More specifically, the present system for producing a mixed gas-oil stream is comprised of a string of tubing extending from the production zone to the surface which has a turbine-compressor system (SPARC) positioned downhole therein. The SPARC is comprised of an upstream separator section; a turbine-compressor section; a downstream separator section; and a means for preventing surging in the turbine-compressor section during the start-up of the SPARC. Basically, the means for preventing surging is comprised of a turbine bypass valve for bypassing the turbine during start-up and a compressor recycle valve for recycling the output of the compressor until surging in the production stream has subsided.
In operation, a well is put on production by opening a choke valve or the like at the surface. As will be understood in the art, normally there will be xe2x80x9csurgingxe2x80x9d in the production stream during the start-up of the well due to alternating slugs of gas and liquid in the stream. If unchecked, this surging can cause significant damage to the turbine and/or compressor thereby shortening the operational lives thereof.
As in prior art SPARC""s of this type, at least a portion of the heavier components, e.g. sand, etc., is separated from the remainder of the production stream as the stream flows through the upstream separator section, e.g. auger separator. These separated components bypass the turbine to thereby prevent erosion within the turbine. However, in the present invention, the turbine bypass valve, when open, allows the separated portion of the stream to be recombined with the remainder of the stream whereby the entire stream bypasses the turbine until surging in the stream has subsided.
As the flowrate of the production stream increases, the change in the differential pressure (i.e. difference between the turbine outlet pressure and the well annulus pressure) acts to close the turbine bypass valve so that only the separated portion of the stream will bypass the turbine. The remainder of the stream, instead of being recombined with the separated portion, will now be directed into the turbine to drive same.
Also, during the start-up period, the open compressor recycle valve will direct the flow from the outlet of the compressor into the downstream separator section which, in turn, separates at least a portion of the gas from the stream and directs this gas into the compressor. The recycle valve remains open until the change in the differential pressure between the outlet pressure of the compressor and the outlet pressure of the turbine causes the compressor recycle valve to close. The closed recycle valve will now direct the flow from the outlet of the compressor (i.e. compressed gas) into the well annulus from which it is injected into an adjacent formation. A check valve is positioned downstream of the compressor to prevent back flow into the outlet of the compressor during the start-up period.